About 40 children die each other from being overheated in cars. Many more than that are taken to emergency rooms with heat stroke or other heat related conditions.
The parents or other care providers responsible are not intentionally cruel people. They simply don't realize how rapidly a car will heat up when parked in the summer sun.
Sometimes the parent means to be only a minute away from the car but a minute is too long and a person can be unavoidably detained.
Leaving the car running with the air conditioner on presents other dangers. Actually, it is against the law to leave a car running without an adult in the car, whether or not there is a child in it.
Leaving the car parked in the shade with the windows down for short periods may be all right for school age children, but infants and toddlers should never be left in a car alone any time of the year.